The freeze-fracture technique has proven useful in the elucidation of membrane structure and membrane events. Freeze-fracture has revealed intramembranous particle specializations in pre- and post-synaptic membranes of mature neuromuscular junctions, and clarified membrane events involved in the release of neurotransmitters. Freeze-fracture has not yet been used to study the development of neuromuscular junctions although such development has been followed in vivo and in vitro by both electrophysiological and thin-section electron-microscopic methods. The results of these techniques, however, have not been correlated by electron-microscopic observation of the same neuromuscular junction which was studied electrophsiologically. Correlated freeze-fracture and thin-section studies of electrophysiologically identified neuromuscular junctions are now possible, with a new method for freeze-fracturing neurites and myotubes developing in tissue culture. Nearly all of the myotubes are fractured, and neurite-myotube contacts identified electrophysiologically can be relocated in the resulting replicas. The proposed research exploits these techniques in an attempt to further clarify the stages in development of neuromuscular junctions, to determine the minimum morphological concomitants of a transmitting junction, and to enhance understanding of the mechanisms involved in insertion and stabilization of intramembranous particle specializations.